The present invention relates to an audible alarm unit using a piezoelectric transducer which has three electrodes, and more particularly to an improvement of in the drive circuit of a self-excited oscillation system for use in an audible alarm unit.
As shown in FIGS. 1 and 2, a piezoelectric transducer 10 comprises a vibration plate 11 made of a material such as brass serving as a first practical electrode secured to an electrode 14 beneath a piezoelectric element 15, a second electrode 12 on the upper face of the element 15, and a third feedback electrode 13 separate from the second electrode 12. As illustrated in FIG. 1, a lead wire is connected to each of the electrodes 11, 12 and 13.
As illustrated in FIG. 3, a conventional drive circuit of the self-excited oscillation type for an audible alarm unit comprises an NPN transistor 20 having a collector 21, an emitter 22 and a base 23, a first resistor 41, a second resistor 42, and the piezoelectric transducer 10. The collector 21 of the transistor 20 is connected to a positive terminal 31 of a direct current power source, the emitter 22 to a negative terminal 32 through the first resistor 41, and the base 23 to the positive terminal 31 through the second resistor 42. Further, the first electrode 11 is connected to the emitter 22, the second electrode 12 is connected to the negative terminal 32 and the third electrode 13 is connected to the base 23. This drive circuit has a disadvantage in that the voltage applied to the piezoelectric transducer 10 cannot be raised above that of the direct current power source since the first electrode 11 and the second electrode 12 of the transducer 10 are connected to the ends of the emitter resistor 41. Therefore, in order to increase the vibration amplitude of the transducer 10 or to increase sound volume, it is necessary to provide a voltage transformer circuit connected to the direct current power source for increasing the voltage, or to provide an alarm casing or horn for improving the acoustical effect, as disclosed in U.S. Pat. Nos. 3,569,963 and 3,890,612.